Current and emerging standards for commercial wireless communication systems based on the Code Division Multiple Access CDMA technology (e.g. IS-95, CDMA2000, UMTS) include a sophisticated scheme for control of reverse link transmit power. The overall power control scheme for the reverse link is organized in two layers. The inner layer is called Reverse Inner Loop Power Control (RILPC), while the outer layer is called Reverse-link Outer Loop Power Control (ROLPC).
The inner layer, RILPC, typically requires the base station receiver to periodically obtain some measure of the SNR being received on a given connection, compare it with a target SNR and send a one-bit feedback to the mobile transmitter. The mobile transmitter lowers (raises) its transmit power by a fixed amount (in the dB domain) if the feedback bit indicates that the measured SNR was higher (lower) than the target SNR. As such, RILPC performs the task of varying the transmit power level of the mobile in such a manner that the SNR at the base station receiver for a given connection is close to the target SNR.
The outer layer, ROLPC, is responsible for varying the above-mentioned target SNR so that an end-user receives their desired quality metrics. In a prevailing fading environment, the target SNR needs to be adjusted continually to deliver the desired quality metrics to the end-user. Typically, the ROLPC monitors the quality metric to obtain a reliable estimate of the same and then raises or lowers the target SNR depending upon whether the estimate is worse than or better than the target quality metric.
One quality metric is bit error rate (BER), which is a “post-decoding” quality metric, i.e., it requires decoding the received signal. This is referred to herein as “post-decoding BER-based ROLPC.” Assuming frame-based transmission, one way of obtaining a BER estimate is to use the log-likelihood ratios for information bits. These log-likelihood ratios are produced by certain decoders as part of the frame decoding process and can be used to estimate an instantaneous BER. The instantaneous BER estimate can be further filtered, or averaged, to obtain a more reliable BER estimate that can be used by ROLPC to adjust the SNR target. Unfortunately, approaches along these lines require strong filtering or averaging over a long period of time, which introduces significant delays in the process of generating BER estimates. Consequently, an ROLPC that uses such techniques to generate BER estimates would be slow to change the target SNR.
An alternative to strong filtering, or averaging over a long period of time, is to use, e.g., the instantaneous BER (generated every frame) or process the instantaneous values through a weak filtering scheme (or average them over a short period of time) before comparing them with the target BER. Such a scheme would be able to quickly change the target SNR.